Cyclosporin A (CsA, 1) is the drug of choice for preventing rejection of transplanted human organs. Mechanism of action studies have shown that CsA binds to cyclophilins A and B (CyP A; CyP B) and that the CsA-CyP complex binds to and inhibits calcineurin (CaN), a calmodulin-dependent serine/threonine protein phosphatase. Although much is known about this property of CsA, only a limited number of CsA analogs have been synthesized and evaluated for immunosuppression, due to the complexity of the total synthesis of derivatives. In addition to immunosuppression, CsA is implicated in several other biological activities including inhibition of HIV replication in acute and chronically infected T-lymphocytes, and binding to hsp7O with dissociation constants near 50 nM. Hsp7O appears to be important in progression of cancer, where it has been implicated in resistance to immunological defense mechanisms and to the onset of drug resistance to certain antitumor drugs. Hsp7O also has been shown to be in the rabies virion and to be a component of the hepatitis B virion. Nothing is known about the effect of CsA structure on these later activities. We have achieved the first synthesis of a CsA analog by solid phase peptide synthesis during the past grant period. We propose to develop the chemistry to carry out solid phase synthesis of CsA itself and its analogs, and to apply this technology to the synthesis of CsA libraries by combinatorial methods. Screening methods for detecting and quantitating binding to cyclophilin, calcineurin, hsp7O, HIV protease and other proteins will be developed. Solution conformations of certain interesting compounds will be determined by NMR. These studies will greatly increase the data-base for correlating the biological activities of CsA with structure, and may provide novel approaches and lead compounds for new therapeutic agents.